Electric wave distorting network



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l STOP ISTARTI asmm R. BEAUFOY ELECTRIC WAVE DISTORTING NETWORK u amnq [START June 30, 1953 Filed Feb. 18. 1952 MARK SPACE- SPACE-- June 30, 1953 R. BEAUFOY 2,644,037

ELECTRIC WAVE DISTORTING NETWORK Filed Feb. 18, 1952 2 Sheets-Sheet 2 Patented June 3Q, 1953 ELECTRKC WAVE DISTURTING NETWORK Raymond Eeaufoy, Taplow, England, assignor to British Telecommunications Research Limited, Taplow, England, a company of Great Britain Application February 18, 1952, Serial No. 272,239

in Great Britain February 22, 1951 The present invention relates to electric wave distorting networks, and is concerned with such networks for use in providing output voltage of substantially rectangular wave form.

A requirement sometimes arises for a network whereby a predetermined one or more of a succession of pulses can be shortened or lengthened. For example in test equipment for use in testing teleprinter apparatus it may be required to shorten or lengthen the duration of some of the elements of an otherwise undistorted teleprinter signal. For example it may be required to shorten all negative, or space, elements in the teleprinter signal, and to lengthen all positive, or mark, elements. On the other hand it may be required to vary the duration of only, say, the start and stop elements of the teleprinter signal.

An object of the present invention is to provide improved apparatus whereby the aforesaid requirements. can be met.

According to the present invention, an electric wave distorting network comprises a capaci tor connected in series with a resistor to the input terminals of the network, a switching device responsive to the voltage across the capacitor exceeding a predetermined amount in one sense to become switched on, and responsive to the said voltage exceeding a predetermined amount in the other sense to become switched oil, and a further switching device adapted to apply and remove a short-circuit across a part of the said resistor automatically to alter the time constant of charge of the capacitor in respect to a predetermined one or more of a succession of voltage pulses applied to the input terminals of the network. The voltage pulses in the succession may, for example, be successive half-cycles of an alternating voltage of rectangular wave form, or successive elements of a telegraph signal.

According to a feature of the invention, the further switch device comprises two rectifiers .connected in series opposition across the resistor, and the junction of the two rectifiers is connected to a tap on the resistor. The tap may be made variable whereby the amount of distortion introduced can be varied. The arrangement according to this feature of the invention functions to distort all positive-going pulses in one sense, and all negative-going pulses in the other sense.

According to a further feature of theinvention the further switch device is adapted to apply or remove the said short-circuit only in respect to voltage pulses of predetermined char- 4 (liaims. (Cl. I'm-70) acteristics. For example where the succession of voltage pulses is provided by a teleprinter signal the pulse of the said predetermined characteristics may be the stop element of the Signal. Distortion of only the stop and start el ments of a succession of teleprinter signals may then be achieved.

The invention will now be described by way of example with reference to the accompanying drawings, in which Figure 1 is an explanatory diagram,

Figure 2 is a theoretical circuit diagram of one embodiment of the invention for use in producing, from undistorted teleprinter signals, teleprinter signals of known distortion, and

Figures 3 and 4 are theoretical circuit diagrams of two parts respectively shown in block form in Figure 2.

Throughout Figure 1 the abscissae represent time and the ordinates represent voltage. V

In Figure 1 (a) the full line it represents an undistorted teleprinter signal comprising a series of signal elements representative of a character. The series comprises a start element extending from t1 to it followed by five alternate mark and space elements which in turn are fol lowed by a stop element extending from it to is. The durations of the start element and the succeeding five mark and space elements are equal and the duration of the stop element is 11/2 times that of the start element and any of the mark and space elements.

The embodiment of the invention now to be described is such that the relative durations of the mark and space elements can be varied and that the duration of the stop and start elements can be varied independently of the other elements of the series.

Referring to Figure 2, two input terminals H and 52 are provided and a resistor l3 and capacitor it are connected in series to the two input terminals II and E2. The terminal I? is earthed. The resistor is is constituted by the fixed element of a potentiometer whose wiper i5 is connected to the moving contact of a switch 96. This switch has two fixed contacts ill and it of which E! is connected to the junction of two rectifiers l9 and 28 which are connected'in series opposition across the resistor 53.

The other fixed contact it of the switch it is connected to the moving contact of a set of relay contacts 25 of a relay 22. The set of contacts 2i includes two fixed contacts 23 and 213 which are connected to opposite ends respectively of tor I4 is connected to the anode of a rectifier 25 whose cathode is connected to a terminal 20 held at a potential of +40 volts. The upper plate of the capacitor It is also connected to the cathode of a rectifier 2'3 whose anode is connected to a terminal 23 held at a potential of -40 volts.

junction of the anode of the rectifier 25 with the cathode of the rectifier 27 is connected to the input of a flip-flop circuit 29. A flip-flop circuit is one which takes up a first stable condition when a positive voltage is applied thereto and takes up a second suitable condition when a negative voltage is applied thereto.

The amplitude of the voltage necessary to cause the flip-flop circuit to change from one of its conditions to the other may be of any convenient value, but in the example now being described it will be assumed that the amplitude necessary is +20 volts and 20 volts. A suitable flip-flop circuit will be described later with reference to Figure 3.

The output of the flip-flop circuit is fed to a telegraph relay 30 which has two fixed contacts 3| and 32 and a moving contact 33, the moving contact being connected to an output terminal 34. A second output terminal 35 is provided this terminal being connected to earth. Fixed contacts 3i and 32 are connected to terminals 31 and 38 which are held at +80 volts and -80 volts respectively.

A character timer 35 (to be described later) is connected between the flip-flop circuit 29 and the relay 22.

If undistorted teleprinter signals as shown in Fig. 1(a) are applied to the terminals H and 12 when the switch i is in the position shown; all positive-going elements of the signal cause the rectifier to become conducting and hence charging current flows from the terminal H through the rectifier l8 and the part of the resistor H) which lies between the wiper l5 and the capacitor IA, into the capacitor l4. All negative-going elements of the signal cause the rectifier 20 to become conducting and hence the charging current for the capacitor is during such elements flows through the part of the resistor 13 which lies between the wiper l5 and the terminal II, and through the rectifier 20.

Thus the time taken for the capacitor 14 to become charged to the voltage at which the flipflop circuit 29 is triggered (that is to say +20 volts and -20 volts in this example) can be made diiferent for negativeand positive-going elements of the signal by appropriate adjustment of the wiper IE on the resistance element i3. The eifect of setting the wiper i5 at the centre or the resistance element 13 would be to provide equal time constants of charge for the capacitor 14 in respect of both negativeand positive-going elements of the signal. 7

It is arranged, as will be described later, that whenever the flip-flop circuit 29 is triggered by negativeand positive-going signal elements the energisation of the relay 30 is reversed and hence the polarity of the output terminal 34 changes from -80 to +80 volts or vice-verse. as the case may be.

Referring now to Figure 1(1)) the full line 39 represents the voltage appearing at the output terminal 34 when the wiper of Figure 2 is set to the central position on the resistance element I3. The time constant of charge of the capacitor M is the same for negativeand positive-going signal elements and hence no distortion appears in the output signal at terminal 34. The.

4 output is merely delayed in time relatively to the signal applied to the input terminals H and I2.

In Figure 1 (c) the full line 40 illustrates the effect of adjusting the wiper 5 to a position on the resistance element [3 at which the time constant of charge of the capacitor i l is shorter in respect to negative-going signal elements than it is to positive-going signal elements, that is to say to a position as shown in Figure 2. The full line '50 indicates the waveform of the voltage appearing at the output terminal 34 of Figure 1 and it will be seen that the negative-going elements (including the start element) are all lengthened, whereas the positive-going elements (including the stop element) are all shortened.

The amplitude of the undistorted teleprinter signal in this example may conveniently be volts. The two rectifiers 25 and 2? biased as described constitute a limiter which prevents the voltage across the capacitor M from exceeding :40 volts and the voltage across the capacitor M is either +40 volts or -40 volts whenever a reversal takes place in the teleprinter signal applied to the input terminals I1 and it. Thus the operation of the network is not dependent upon the capacitor I l becoming fully charged during each mark and space element and hence a long time constant can be employed. The time taken for the voltage across the capacitor Hi to reach -20 volts from +40 volts and to reach +20 volts from -40 volts during operation is a function of the amplitude of the signals applied to the input terminals and the time constant of the capacitor and resistor connected in series therewith. Thus for a given amplitude of applied signal the amount of distortion introduced can be expressed in terms of the time constant and hence a scale associated with the wiper 15 can be calibrated in percentage distortion.

The distortion produced in the arrangement as so far described is usually termed bias distortion.

The network of Figure 2 also enables distortion of only the start and stop elements of the signal to be efiected, the remaining elements being allowed to remain undistorted. This facility is obtained by moving the arm of the switch 16 to the fixed contact i8 thereby rendering the rectifiers i9 and 20 inoperative and rendering the relay 22 operative.

The character timer 30 is made such that the relay 22 is energised for the duration of each stop element in the applied teleprinter signal and for the remainder of each signal is die-energised. Thus the time taken for the capacitor i l to be charged to -20 volts from +40 volts (which is the voltage across the capacitor during the stop element) can be varied by varying the position on the wiper l5 and the resistance element l3. Thus, for example, with the wiper I5 in the position shown and the relay energised as shown, the part of the resistance element l3 which lies between the wiper i5 and the capacitor i4 is short-circuited by the relay contacts 22 and hence the time constant of charge of the capacitor I4 is relatively small. Throughout the remainder of the signal the part of the resistance element 13 between the wiper i5 and the terminal II is short-circuited and the time constant is relatively large. The time taken therefore for the capacitor M to be charged to 20 volts from +40 at the end of a stop element is less than at the end of any other positive-going elements in the signal. The waveform of the voltage appearing at the output terminal 34 is therefore as shown by the full line ll in Figure 1(cZ) from which it will be seen that the stop element is shortened and the start element is lengthened but all other elements are of the same length as in Figure 1(a) Referring now to Figure 3, this is a circuit diagram including a suitable fiip-flop circuit for use at 29 in Figure 2. In Figure 3 the upper plate of the capacitor it is connected to the control grid of a triode valve connected as a cathode follower and having a cathode load resistor it. This cathode follower serves to prevent the iiip=- flop circuit from affecting the time constant of charge of the capacitor iii. The cathode of the valve 32 is connected through a resistor 2 to the control grid of a further triodevalve t5 whose cathode lead includes one winding of the relay til. The second winding of the relay 3B is in the cathode lead or" a further triode valve 16 and a cathode load resistor All is common to both triodes 35 and Alt. The two triodes i5 and it are provided with anode load resistors E8 and 49 respectively and anode voltage for the three triodes is provided from a terminal l-lT-l-l. The control grid of the valve 35 is connected to the anode of the valve 56 through a resistor 5i? which is shunted by a capacitor 55 and the control grid of the valve 56 is connected ot the anode of the valve t5 through a resistor E52 shunted by a, ca pacitor 53. The control grid of the valve it is also connected through a resistor Ki l to a tap on the common cathode resistor ii.

In operation the mean potential of the control grid of the cathode follower valve 42 is zero and goes alternately positive and negative. The cathodes of the valves 45 and l -'5 are positive relatively to earth because of the voltage drop across the common cathode resistor 1 When the control grid of the valve 32 is made positive the cathode thereof becomes more positive by a like amount and when the control grid of the valve 32 becomes negative by more than a predetermined amount the anode current in the valve 52 is reduced to zero. Thus the voltage between the control grid and cathode and the valve 35 can readily be made to change from a positive to a negative value.

The circuit is arranged in known manner such that when the control grid of the valve 52 reaches volts the valve 55 becomes rapidly conducting and the valve :26 becomes non-conducting, and that when the voltage at the control grid of the valve :32 falls to 20 volts the valve 55 rapid= 1y becomes non-conducting and the valve at be-- comes conducting;

Thus current flows firstly through one of the windings of the relay 3d and then through the other, whereby the waveforms as shown in Figure 1 are produced at the output terminal 5d.

Referring now to Figure 4, this is a circuit diagram including. a suitable character timer for use at 36 in Figure 2. The flip flop circuit 2% is connected through a resistor 55 to the control grid of a triode valve 52' provided with a grid leak 58. The control grid of the triode 5'! is connected through a resistor 59 to the anode of a further triode valve 68 and anode voltage for the two valves 5? and 5%? is supplied from a ter-- minal HT-l-Z through resistors 8i and 52 respectively. A connection is made from a tap on the resistor 5! through a capacitor 63 to the control grid of the valve to which is provided with a grid leak 6 Two resistors 55 and 6%? are connected in the cathode leads of the two valves 5? and Bil respectively and the relay winding 22, corresponding to that shown in Figure 2, is connected betweenthe cathodes of the two valves 51 and til.

it will be seen that the character timer of Figure 4 is in the form or a well known multivibrator circuit. This circuit is such that the valve 52 is normally non-conducting and the valve til is normally conducting. On the application of a positive pulse to the control grid of the valve 5? the anode current in this valve rises rapidly. This causes rapid fall in the anode voltage of the valve 5] and. hence the control grid of the valve (ill is driven negatively and hence the anode potential of the valve 6% increases. This increase is transmitted to the control grid of the valve 5? causing a further rise in the anode current of this valve. This action is cumulative and a condition is rapidly reached in which the valve 89 is non conducting and the valve 5'5 is highly conducting. The capacitor 63 then begins to discharge at a rate determined mainly by the values of the capacitor 63 and the resistor 64 and by the setting of the tap on the resistor El. After an interval of time which may be varied by varying the position of the tap on the resistor iii the valve 69 becomes conducting once more. The process is then reversed and the valve 57 rapidly becomes non-conducting. A further positive pulse is then required at the control grid of the valve 5? to repeat the cycle of operations.

In the present example it is arranged that the time taken for the valve 51 to become conducting once more following the application of a positive pulse to the control grid of the valve 57, is ual to six mark and space elements, that i to say is equal to the duration of a complete character less the duration of the stop element.

The polarities of the signals applied to the control grid of the valve 51 are reversed relatively to those of the signals applied to the input terminals H and it of Figure 2 and hence each stop element applied to the terminal 5? is nega tive-going and each start element is positivegoing. Thus, assuming that the cycle of operations of the circuit of Figure 4 commences at the beginning of a start element the valve 57 becomes conducting oncemore just at the commencement of the next succeeding stop element. The valve 5? remains conducting for the duration of this stop element and the next cycle of operations of the circuit of Figure l begins at the end of this stop element. This is the mode of operation taken up by the circuit of Figure l, no special steps being necessary to ensure this synchroni sation. This follows from the fact that the only elements which are identical in all teleprinter signals are the start and stop elements and hence if the circuit of Figure l is not in synchronism immediately afterswitching on a succession of varying teleprinter signals causes the circuit to pull into the synchronisrn very quickly. 7

Although the invention has been described with reference to a succession of teleprinter sig-- nals following immediately after one another, it will be understood that the embodiment described may also be used without modification for use in producing distortion of isolated teleprinter signals.

I claim:

1. An electric wave distorting network c0rnprising a ca acitor connected in series with a resistor to the input terminals of the network. a switching device responsive to the voltage across the capacitor exceeding a predetermined amount in one sense .to become switched on and responsive tothe said voltage exceeding a predetermined amount in the other sense to become switched off, anda further switching device adapted to apply and remove a short-circuit across a part of the said resistor automatically to alter the time constant of charge of the capacitor in respect to a predetermined one or more of a suc cession of voltage pulses applied to the input terminals of the network.

2. A network according to claim 1 for distorting all negative-going pulses in one sense and all positive-going pulses in the opposite sense, wherein the said further switch device comprises two rectifiers connected in series opposition across the resistor, andthe junction of the two rectifiers is connected to a tap on the resistor.

3. A network according to claim 1 suitable for use in distorting selected ones of the said pulses, wherein the said further switch device is adapted to apply or remove the said short-circuit in response to voltage pulses of predetermined characteristics.

4. A network according to claim 3 for use in distorting the start and stop elements of teleprinter signals, wherein the said further switch device comprises a, relay having a moving contact connected to a tap on the resistor and having two fixed contacts connected to the two ends of the resistor respectively, and the operating winding of the relay is energised from a timing device adapted to cause operation of the relay in one sense during each stop element and to cause operation of the relay in the other sense throughout the remainder of each teleprinter signal of a succession of teleprinter signals applied to the input terminals of the network RAYMOND BEAUFOY.

References Cited in the file of this patent UNITED STATES PATENTS umber Name Date 2,030,814 Erickson et al Feb. 11, 1936 2,568,019 Martin Sent. 18, 1951 

